Osmotically driven membrane processes are capable of treating high fouling solutions in an energy efficient manner by using a chemical energy gradient between two solutions to drive water flux across a membrane. Osmotic pre-treatment processes utilize a draw solution with a high osmotic potential, or osmotic pressure, relative to the feed solution to provide a driving force for water transport across a membrane. As relatively pure water flows across the membrane, it dilutes the draw solution.
In an osmotically driven membrane water purification system, the draw solution must then be re-concentrated, or the draw solute recovered in some way for recycling. Typical systems employ either desalination equipment such as reverse osmosis, distillation, or other salt separation technique, or other methods for draw solute recovery such as thermally switching salts. Typically, these systems are controlled as two separate sub-systems, an osmotic system and a re-concentration system, with a buffer volume between them and two separate pumping systems. The separate pumping systems regulate the flowrate and pressure of each stream.
The overall rate of production in an osmotic system is dictated by the flow of water across the osmotic membrane, which is determined in large part by the differential of the draw solution osmotic pressure to the feed solution osmotic pressure. In traditional systems, the draw solution concentration is controlled by adding solute to the draw solution with an injection pump and a draw solution buffer tank.
In all cases, the draw solution osmotic potential must be higher than the feed to be treated. In some cases, high feed water concentrations may necessitate draw solution concentrations that are higher than typically treatable to reconcentrate in lower energy desalination technologies, such as reverse osmosis (RO).
The energy recovery pumps generally used in conventional RO systems can also be operated in a different way in order to be utilized for pressure retarded osmosis (PRO) systems. In PRO systems, a high osmotic potential draw solution is diluted by a low osmotic potential feed solution. The osmotic driving force is partially offset by pressurizing the draw solution, but water flux is still in the direction of the draw solution. The excess water in the draw solution is relieved through an energy generating device such as a turbine.